1.1 Field of the Invention
This invention relates to a cortical bone implant for use in cervical Smith-Robinson vertebral fusion procedures, as well as methods for the manufacture and use thereof. Furthermore, this application relates to an assembled implant comprised of two or more individual segments fastened together.
1.2 Background Art
Since at least the mid to late 1950's anterior cervical spinal fusions have been performed in order to alleviate chronic neck, aim and shoulder pain caused by trauma, disc herniation, or spondylosis (Robinson and Smith, 1955; Smith and Robinson, 1958). The classic procedure referred to as the Smith-Robinson cervical fusion employs a horseshoe-shaped graft to promote vertebral fusion (Robinson et al., 1962). The Cloward technique employs a cancellous bone dowel (Cloward, 1958), and the Bailey-Badgley procedure uses a strut (Bailey and Badgley, 1960). In a study comparing the compressive load capacity of the various implants used according to these procedures, it was found that the Smith-Robinson graft could sustain loads up to 344 N, the Cloward dowel could sustain loads of up to 188 N, and the Bailey-Badgley type could sustain loads up to 195 N, (White and Hirsch, 1972). In a modified Smith-Robinson procedure, the horseshoe-shaped implant is inserted with the cortical end of the implant located posteriorly, which has been reported to increase the fusion rate while decreasing the graft extrusion and collapse sometimes experienced with the Cloward dowels (Whitecloud and Dunsker, 1993). However, in a recent study evaluating the success and relief rates achieved according to these procedures, it was found that less than 100% success rate (fusion, patient improvement and absence of complications) was achieved, regardless of which method or implant was used (Grooms et al, 1996).
U.S. Pat. No. 5,306,309, discloses a spinal disk implant comprising a solid body of biocompatible synthetic material arranged to define a right-rectangular solid having two opposed side faces and two opposed transverse faces, including a convexly curved anterior face and a posterior face, for implantation in the intervertebral space. The discussion of vertebral and intervertebral morphology is hereby incorporated by reference.
U.S. Pat. No. 5,609,635, discloses a lordotic interbody spinal fusion implant comprising a wedge shaped metallic cage for insertion into the intervertebral space.
U.S. Pat. No. 5,306,307, discloses a ceramic spinal disk implant having a serrated edge.
None of these references disclose a cortical bone intervertebral implant having a substantially “D”- or bread-loaf-shaped structure having a canal into which osteogenic, osteoinductive, or osteoconductive materials may be packed, which sustains spinal loads, and which is remodeled into the spine in the course of fusion. Accordingly, the present invention addresses the need in the art for improvements to both the implant and the avoidance of post-surgical complications from anterior cervical fusions. The present invention provides a new cortical bone implant for use in achieving anterior cervical fusions when implanted according to the Smith-Robinson procedure. In addition, in view of the peculiar characteristics of bone, the present invention comprises unique methods and apparatuses for the manufacture of the substantially “D”-shaped cortical bone implant.